This application is a 371 of PCT/CA99/00121, filed Mar. 4, 1999 which claims benefit for U.S. Ser. No. 09/036,389, filed Mar. 6, 1998, now U.S. Pat. No. 6,74,773.
The present invention relates to porous electrocatalyst particles which may be incorporated into an electrochemical cell. More particularly, the invention provides a method for improving electrochemical fuel cell performance by impregnating porous electrocatalyst particles and incorporating such particles into an electrochemical fuel cell.
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. In addition to electrocatalyst the electrodes may also comprise an electrically conductive substrate upon which the electrocatalyst is deposited. The electrocatalyst may be a metal black (namely, a substantially pure, unsupported, finely divided metal or metal powder), an alloy or a supported metal catalyst, for example, platinum on carbon particles.
A solid polymer fuel cell is a type of electrochemical fuel cell which employs a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d). The MEA comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers.
A broad range of reactants can be used in electrochemical fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute oxygen stream such as air.
The electrochemical oxidation which occurs at the anode electrocatalyst of a solid polymer electrochemical fuel cell, results in the generation of cationic species, typically protons, and electrons. For an electrochemical fuel cell to utilize the ionic reaction products, the ions must be conducted from the reaction sites at which they are generated to the electrolyte. Accordingly, the electrocatalyst is typically located at the interface between each electrode and the adjacent electrolyte.
Effective electrocatalyst sites are accessible to the reactant, are electrically connected to the fuel cell current collectors, and are ionically connected to the fuel cell electrolyte. For example, if the fuel stream supplied to the anode is hydrogen, electrons and protons are generated at the anode electrocatalyst. The electrically conductive anode is connected to an external electric circuit which conducts an electric current from the anode to the cathode. The electrolyte is typically a proton conductor, and protons generated at the anode electrocatalyst migrate through the electrolyte to the cathode. Electrocatalyst sites which are ionically isolated from the electrolyte are not productively utilized if the protons do not have a means for being ionically transported to the electrolyte. Accordingly, coating the exterior surfaces of the electrocatalyst particles with ionically conductive coatings has been used to increase the utilization of electrocatalyst exterior surface area and increase fuel cell performance by providing improved ion conducting paths between the electrocatalyst surface sites and the electrolyte.
A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. Increasing effective utilization of the electrocatalyst surface area enables the same amount of electrocatalyst to induce a higher rate of electrochemical conversion in a fuel cell resulting in improved performance.
Electrocatalyst materials used in electrochemical fuel cells typically comprise noble metals such as platinum. These materials are expensive, so in addition to improving performance, the present method may also be used to reduce the amount of noble metal used in an electrochemical fuel cell thereby reducing material costs.
U.S. Pat. Nos. 5,186,877 and 5,346,780, disclose methods of coating the exterior surfaces of electrocatalyst particles with a proton conductive film such as an ionomer to improve ionic conductivity between the electrocatalyst and the electrolyte. Known methods of applying exterior coatings to electrocatalyst particles include brush coating and using electrocatalyst/ionomer inks.
In conventional methods, the amount of ionomer coating on the electrocatalyst particles is controlled because too much ionomer can result in reduced electrocatalyst utilization due to poor electrical conductivity and/or reduced reactant accessibility to the electrocatalyst sites. Higher reactant accessibility is preferred to allow the reactants to access the electrocatalyst surfaces at a rate which will sustain the desired electrochemical reaction. As described above, too little ionomer can result in reduced proton conductivity and reduced electrocatalyst utilization.
A problem with conventional methods of coating electrocatalyst particles with ionomers is that they do not effectively impregnate the pores of electrocatalyst particles, especially where pores have aperture sizes less than 0.1 micron (hereinafter defined as micropores). Interior surface areas are defined as those surface areas which form the walls of cracks or pores in the electrocatalyst particles. When porous electrocatalyst particles which have been coated by conventional methods are used in electrochemical fuel cells, a significant portion of the electrocatalyst interior volume and the corresponding interior surface areas are not utilized.
Another problem with conventional methods is that the coatings applied to the electrocatalyst particles may actually obscure or block the pore openings. If micropore openings are blocked, reactants may be prevented from entering the micropores and accessing interior surface areas of the electrocatalyst particles, reducing electrocatalyst utilization and diminishing fuel cell performance.
Complete utilization of the electrocatalyst particle surfaces is limited in part by mass transport limitations within the pores of porous electrocatalyst particles. Thus there is a need for a method of improving the mass transport properties to enhance the reactant accessibility and ionic conductivity within the pores of the electrocatalyst particles.
The invention provides a method of treating porous electrocatalyst particles and the use of the treated particles for improving performance in an electrochemical fuel cell. The treatment of the electrocatalyst particles comprises impregnating pores of the electrocatalyst particles with an impregnant, and in particular, impregnating micropores having an aperture size less than 0.1 micron. The method of improving electrochemical fuel cell performance comprises incorporating the treated porous electrocatalyst particles in an electrochemical fuel cell at an interface between an electrode and an electrolyte in the fuel cell. The impregnant is selected to improve reactant and/or ion transport properties within the micropores of the electrocatalyst particles.
An impregnant is defined as a material which has been deposited within the micropores of electrocatalyst particles. Such impregnants are preferably introduced into the micropores as a liquid, but may be introduced in any phase depending upon the properties of the particular impregnant. The impregnant may be a homogenous material or a mixture of different materials which may be in different phases. For example, the impregnant may comprise a solid material which is suspended in a liquid material.
The impregnant preferably comprises an ion-conducting material. Many types of ion-conducting materials are suitable, so long as they are compatible with the operating environment and other components of electrochemical fuel cells. For example, the ion-conducting material may be an inorganic acid, such as H3PO4, and HNO3, or an organic acid, such as fluorinated organic acids like CF3COOH, and CF2SO3H, or a polymer, such as perfluorosulfonic acid (xe2x80x9cNAFION(copyright) brand perfluorosulfonic acid (hereinafter NAFION(copyright))xe2x80x9d) or a sulfonated trifluorostyrene based polymer.
If the electrocatalyst is to be used at the cathode of an electrochemical fuel cell, the impregnant preferably comprises a material with oxygen permeability greater than that of water. Dilute oxygen is the oxidant which is typically used in acid electrochemical fuel cells. Improving oxygen permeability within a fuel cell results in improved mass transport properties for transporting the oxidant to the cathode electrocatalyst sites. Accordingly, impregnating the electrocatalyst micropores with a material which has an oxygen permeability greater than that of water, improves the transport of oxygen to interior electrocatalyst surface areas and increases utilization of interior electrocatalyst sites. Thus the impregnant may comprise an organic fluid which has these properties, such as, for example, organic fluids selected from the group consisting of hydrocarbons, fluorocarbons, and non-halogenated oils. Where the organic fluid is a fluorocarbon, it may be a perfluorocarbon, such as perfluorotripropylamine, cis-perfluorodecalin, trans-perfluorodecalin, perfluoro-1-methyl decalin, perfluorotributylamine, perfluoroisopentyltetrahydropyrane, perfluoro-N,N-dimethylcyclohexylamine, or perfluoroperhydrophenanthrene. Where the organic fluid is a non-halogenated oil, it may be a silicone oil or a mineral oil.
An embodiment of the method of improving electrochemical fuel cell performance comprises incorporating porous electrocatalyst particles in an electrochemical fuel cell at an interface between an electrode and an electrolyte wherein the particles have been subjected to a vacuum to impregnate pores in the particles with an impregnant.
A preferred method of impregnating porous electrocatalyst particles to deposit an impregnant within micropores thereof, comprises contacting the electrocatalyst particles with an impregnant at a pressure above or below atmospheric pressure. For example, the electrocatalyst particles may be subjected to reduced pressure or a vacuum to evacuate the pores thereof and then contacted with the impregnant, or the electrocatalyst particles may be subjected to a vacuum in the presence of the impregnant. Alternatively, elevated pressure may be used to facilitate penetration of the impregnant into the micropores.
The method may also comprise subjecting the electrocatalyst particles sequentially to both a vacuum and an elevated pressure (above atmospheric pressure) to enhance the penetration of the impregnant within micropores. The method may be applied to particles of carbon-supported electrocatalysts or to unsupported electrocatalysts.
The product of the present electrocatalyst treatment method may be incorporated into an electrochemical fuel cell. An electrochemical fuel cell comprises an anode, a cathode, and an electrolyte disposed between the anode and the cathode. The treated electrocatalyst particles are preferably disposed at the interface between the electrolyte and either or both of the anode and cathode. Conventional methods of coating the electrocatalyst particle exterior surface areas, such as, for example, NAFION(copyright) brush coating of the electrodes, may be used in combination with the present impregnation of the pores in the electrocatalyst particles.
Several advantages may be achieved by practicing the described method. For example, compared to electrochemical fuel cells which incorporate untreated electrocatalyst particles, improved electrochemical fuel cell performance is achieved by incorporating electrocatalyst particles which have been treated in accordance with the described impregnation method. It is believed that the impregnant improves utilization of interior electrocatalyst surface areas within the micropores by improving mass transport properties within the micropores for reactants and/or products of the electrocatalyst induced reactions.
A benefit of increasing electrocatalyst utilization is that it provides options for improving electrochemical fuel cell performance and/or reducing material costs by using lesser amounts of electrocatalyst.
Another advantage is that, in addition to increasing electrocatalyst utilization, the method can also be used to improve other aspects of electrochemical fuel cell performance. For example, an impregnant may be selected to impart additional desirable properties such as preferential reactivity under cell reversal conditions, selective oxidation of electrocatalyst poisons, hydrophobicity, and hydrophilicity. For example, an impregnant could comprise an organo-metallic macrocyclic such as cobalt phthalocyanine which may be preferentially reactive under cell reversal conditions to prevent permanent damage to fuel cell components. A hydrophobic material such as polytetrafluoroethylene (PTFE) may be used as the impregnant or as part of an impregnant mixture to reduce water penetration of the electrocatalyst micropores, which might inhibit the access of reactants to the interior sites.
The desired properties within the electrocatalyst micropores of a particular electrochemical fuel cell may depend upon a number of factors such as, for example, the type of reactants and the anticipated operating conditions. The method and product of the invention provide a means for modifying the properties within the electrocatalyst micropores.